1. Field of the Invention
The present invention relates to a coating process. More particularly, it relates to a process for consecutively coating both sides of a continuously travelling band-shaped support (hereinafter referred to as a "web"), which comprises applying a coating solution to the one side of the web, and applying another coating solution to the other side of the web while the first coating layer is still in a non-dry state.
2. Description of the Prior Art
In the process of consecutively coating both sides of a web by providing a coating layer on the one side thereof and applying another coating layer to the other side of the web while the first coating layer still is in a non-dry state, the second coating must be conducted, after providing the first coating layer, without disturbing the first coating layer.
A process for satisfying such requirements for consecutive coating of both sides of a web, which has been suggested, is a gas-supporting coating process wherein the second coating is conducted while supporting the first coated side of a web in a non-contact manner (e.g., as disclosed in Japanese Patent Publication Nos. 17,853/74, 38,737/76, etc.). In these processes, second coating is conducted by bead coating, extrusion coating, etc. while supporting the first coated side of a web with the pressure of a gas flow. However, no matter how accurately the gas pressure may be controlled, it is extremely difficult to avoid minute vibrations of the web position due to change in gas pressure, etc. Therefore, various problems have occurred. For example, in bead coating, extrusion coating or the like which has been used in combination with the technique of supporting a web by the pressure of a gas flow, the relative positionwise relationship between a coating apparatus and a web greatly influences the thickness of a coating layer, and hence minute vibrations as described above of the web position have naturally resulted in stepwise coating unevenness. Accordingly, the process of combining bead coating or extrusion coating with the technique of supporting a web with the pressure of a gas flow has not necessarily provided industrially satisfactory results.
To improve the conventional gas-supported coating process, it can be considered to conduct the second coating using a process which does not require the web to be backed up or supported; with a typical example of such a process being bar coating. In bar coating, a coating solution is applied in an excess amount to a web by kiss coating, and the excess coating solution is removed using a wire bar or grooved bar, which is stationary or is intermittently or continuously rotated in an opposite direction to the travelling direction of the web at a slower peripheral velocity than the travelling velocity of a web to thereby meter the coating amount to a desired level. This process usually does not require the web to be backed up at the coating and metering sections. However, when a web is not well supported, this process also tends to generate stepwise coating unevenness or longitudinal streaks due to a flapping of or wrinkles in the web, and hence restraining rolls or the like are usually disposed in order to prevent the web from flapping and maintain the planar state of the web.
In the process of consecutively coating both sides of a web, however, it is difficult to support a web using restraining rolls or the like. Thus, in order to minimize flapping or wrinkling of the web in the coating section and metering section, it is considered necessary to support the web with a pressurized gas in the coating and metering sections. According to investigations which have now been made, it has been observed that, when bar coating is conducted while supporting a web with a pressurized gas, stepwise coating unevenness due to minute vibrations of the web position is reduced as compared with bead coating or extrusion coating, whereas intimate adhesion between the web and the coating roll is deteriorated particularly in the kiss coating section with longitudinal streaks being generated on the coated surface. Such longitudinal streaks may be attributed to longitudinal wrinkles formed on the web. That is, even the techniques of supporting the web with a gas under pressure fails to completely remove longitudinal wrinkles formed in a web. These longitudinal wrinkles are removed to a considerable extent in the bar-metering section, since a web tends to conform to the surface of the bar due to the small diameter of the bar, thus serious influences on the coated surface are not exerted. However, in the kiss coating section, a web conforms with difficulty to the surface of a coating roller since the diameter of the coating roller is much greater than that of a bar and, as a result, longitudinal wrinkles in a web are believed to be removed with difficulty and longitudinal streaks are formed on the coating layer. Further, when the technique of supporting the web with a gas under pressure is combined with the bar coating process, the web must be supported by a gas under pressure in both the kiss coating section and the bar metering section, and thus a large space is necessary. Therefore, this approach has not been employed industrially in consecutively coating both sides of a web.